The present invention relates generally to the hot-cracking sensitivity of thin sheet metal during welding to determine the weldability of the metal. More particularly the present invention is directed to the method and apparatus for applying preselected stress loadings to the sheet metal prior to and in the direction transverse to the direction of welding in order to determine the weldability of the sheet metal by evaluating the sensitivity of the metal to hot cracking at various stress loadings.
The weldability of sheet metal formed of essentially the same alloy composition has been found to differ from heat-to-heat and by using different welding process variables such as current, arc spacing and welding speed so as to produce various welding defects. These welding defects include hot cracking with the cracks ranging in size from microcracks in the welding area to separation of the sheet metal along the weldment. The tendency for hot cracking during welding is primarily influenced by the metallurgical characteristics of the alloy. The formation of a crack in a weldment requires that material comprising the weldment exhibit a phase or combination of phases and possess a limited capacity to tolerate strain within some critical range of temperatures and that the strain imposed upon the weldment by the combined action of thermal and restraint conditions within this critical range of temperatures exceeds the strain tolerance of the composite microstructural region. Other weldment defects such as cold cracking, slagging, poor fusion, porosity, and the like are primarily caused by welding process variables.
In determining the weldability of sheet metal, it has been found that the sensitivity of the sheet metal to hot cracking during welding to be a satisfactory technique for evaluating the weldability of sheet metal. Ideally a hot-cracking test for determining the weldability of sheet metal would have the capability to show a direct correlation with the actual fabrication and service behavior and the reproducibility of results with freedom from variations due to operator skills. The criteria for such a test should also include a high level of sensitivity to small changes in test variables and the ability to show the effects of several welding variables as well as applicability to all welding processes utilized for sheet metal.
Several techniques have been previously utilized for determining the weldability of sheet metal formed of various alloy compositions. Generally, the sensitivity of the sheet material to hot cracking during welding has been determined by employing a so-called "VARESTRAINT" test. This test is a variable restraining test developed by Rensselaer Polytechnic Institute, Troy, N.Y. and comprises mounting a rectangular specimen of sheet metal in a fixture in a cantilever manner. The specimen was then autogenously welded from the unsupported end and as the weld pool passed a certain longitudial point, a load was rapidly applied to the specimen to suddenly bend the specimen to conform to the curved surface of an underlying die block. This augmented strain applied during the welding is used to stress the specimen since the inherent restraint in the specimen is relatively small and too low to induce hot cracking. This level of augmented strain required to cause hot cracking in the speciman with a particular set of welding parameters provided an accurate measurement of the hot-cracking sensitivity. The variation in specimen heats as well as welding parameters cause cracking at different levels of augmented strains so as to provide reproducibility and quantitative methods for determining the weldability of various alloy compositions. Further details of this previous technique and variations thereof as well as details relating to hot-cracking characteristics of metals are set forth in the Welding Research Council Bulletin entitled "The Varestraint Test" by C. D. Lundin et al, Bulletin 280, Aug. 1982, available from the Welding Research Council, United Engineering Center, 345 East 47th St., New York, N.Y., 10017. This publication is incorporated hearin by reference.
Other techniques used for evaluating the hot-cracking sensitivity of sheet metal for determining the weldability of the metal include the so-called Lambert test and the ORNL test. Generally, in the Lambert test, a coupon composed of a prestressed sheet (100 mm wide.times.200 mm long) of say, type 304L stainless steel is butt welded to a sample of the same dimensions of the material to be evaluated. The overall length of the fabricated coupon is 400 mm long. The type 304L sheet is used as a standard and the test procedure consists of side-by-side autogenous welds provided by a gas-tungsten-arc (GTA) welding process which provides a moving pool of weld metal along the length of the test coupon. About 12 to 16 weld passes are used on each weld pad with two weld pads being provided per coupon. A dye penetrant is then used to indicate the presence of cracks in the sample. Details of the Lambert test are discussed in the publication "The Effect of Phosphorus, Sulfur, and Ferrite Content on Weld Cracking of Type 309 Stainless Steel," by J. A. Brooks and F. J. Lambert, Jr., Welding Journal, Volume 57(5), pages 139 to 143 (May 1978).
The ORNL test is described in assignee's U.S. Pat. No. 4,499,758 and generally comprises the mounting of a disc-shaped specimen in a test fixture. Two circular autogenous welds are made using the gas-tungsten-arc welding process. The welds are of different diameters and establish increasing restraint levels in the specimen. The specimen is then removed and turned 180.degree. and a weld procedure for providing welds of the same diameter is repeated. The earlier weld procedure renders the microstructure of the material susceptable to cracking and also increases the residual stress within the material that could enhance the material suscepibility for cracking. This restraint may be varied to the extent that the top plate is clamped down.
While prior art techniques such as described above provide satisfactory evaluation of hot-cracking sensitivity for determining the weldability of various alloy compositions there are some shortcomings or drawbacks which detract from their usefulness. For example, with previous techniques, such as the Varestraint test, as described above, thin sheet material less than about 0.125-inch thickness could not be ranked while the Lambert weldability test, which was capable of ranking thin sheet material, was highly operator sensitive as to introduce an undesirable level of errors from test-to-test and operator-to-operator. In the ORNL test, as described above in the aforementioned patent, the problems include limits on the restraint which can be applied (limited to self-restraint of the specimen) and the non-quantitative measure of cracking sensitivity which results.